This invention relates generally to the field of noise attenuation and deals more particularly with a sound attenuator used to prevent low frequency noise from being transmitted from equipment rooms and other noisy areas into occupied spaces.
In the air distribution systems of office buildings, commercial buildings and other large building, the air ducts and particularly the return ducts are a major source of low frequency noise. This problem is particularly noticeable in occupied spaces located near an equipment room. The air handling and refrigeration machinery generates low frequency sounds which originate in the equipment room and travel into the occupied space via the return air path. The occupied spaces closest to noise generating equipment are most subject to this type of noise problem.
High frequency noise (above about 500 Hz) can be successfully attenuated by conventional techniques such as the use of duct liners and suitable terminal units. Lower frequencies in the second, third and fourth octave bands (125 Hz, 250 Hz and 500 Hz) can be effectively absorbed by modules of the type shown in U.S. Pat. No. 4,068,736 to Dean et al. This type of sound absorber is particulrly effective in the third and fourth octave bands and also functions better than other devices in the second octave band. However, in the first octave band (63 Hz), neither the patented module described in the aforementioned Dean et al. patent nor any other known sound absorber provides more than a small amount of sound absorption, and frequencies below about 100 Hz remain a significant problem.
Sound absorbers have been used in the past both in through-the-wall applications and in-duct applications, although in-duct installations have been much more common. It has been assumed that the attenuation does not differ significantly whether the sound absorber is used in a through-the-wall application or an in-duct application, so long as proper design parameters are used. This assumption is valid at high frequencies and also at low frequencies in most cases. However, we have found that a through-the-wall application can achieve some additional and unexpected attenuation at low frequencies by taking advantage of a phenomenon known as "end reflection loss". This phenomenon results in the reflection of a certain amount of sound back into a duct when a sound wave passes from the duct into a much larger space such as a room. However, as indicated on page 32.11 of the 1984 ASHRAE Guide on Systems in the section on "Sound and Vibration Control", this occurs only if the duct termination is preceded by a straight section of ductwork which is 3 to 5 duct diameters long and which lacks a diffuser or grille.